Coexistence and the niche in a nurse-cactus interaction: is cyclic dynamics justified?
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1. The interaction between the nurse plant Larrea tridentata and the cacti Cylindropuntia leptocaulis is thought to follow cyclical-replacement dynamics. However, the required changing nature of their interaction, from facilitation to competition, has not been investigated through their full life cycle. 2. In order to test the hypothesised cyclical dynamics, we compared the demography of four sub-populations (LA = Larrea associated to Cylindropuntia; CA = Cylindropuntia associated to Larrea, LS = solitary Larrea; CS = solitary Cylindropuntia) in the Mapimí Biosphere Reserve, Mexico, over the period 2008-2015. This allowed us to compare their demography at varying levels of the generally low and unpredictable limiting resource, water. Given the protection afforded to C. leptocaulis seedlings by L. tridentata and their presumed increasingly competitive effect on L. tridentata, which would justify the expectation of cyclic replacement dynamics, we expected to find a zero or positive per capita rate of population growth (r≥0) in CA and LS, and a negative one in CS and LA. 3. The overall r over the study period was close to equilibrium (r=0) for both LA and LS, with no significant difference between them. In contrast, and as expected, r of CA was consistently and significantly larger than that for CS. In both species, λ (=er) was positively correlated with annual rainfall with no significant difference between LA and LS and a significant difference between CA and CS in this relationship. L. tridentata can exist alone and coexist with C. leptocaulis across the levels of precipitation experienced at the study site, while C. leptocaulis cannot persist in the absence of L. tridentata. The cyclical dynamics of replacement does not occur in this system and, given the static nature of the evidence on which the original conclusion was based, it is unlikely to exist as a general rule across the species’ geographic range. 4. Synthesis: Our findings stress the importance of elucidating the demographic mechanisms that allow species competing for a common limiting resource to coexist when the resource varies temporally, and expose the difficulty of identifying a species’ ecological niche in the absence of detailed, long-term demographic information.
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